The present invention relates to a machine for the production of receptacles of plastic material comprising a chassis and arranged along a transport path, at least one heating device and a device for moulding by stretching and blowing, the receptacles being formed from preforms disposed on supports, the device for moulding by stretching and blowing comprising a blowing member connected to a source of fluid under pressure by means of an electro-valve and a stretching rod adapted to be displaced by actuating means comprising a motor connected to a control installation comprising first regulating means to program and control the displacement of the stretching rod and second regulating means to program and control the pressure increase applied to the preforms. It also relates to a corresponding process.
The document WO97/13632 discloses a stretch blow moulding machine in which the stretching rod is displaced by the introduction of air under pressure into chambers of a pneumatic jack. This type of actuating means does not permit precise adjustment of the movements of the rod and easy modification of the laws controlling the displacements of this rod. There exists also machines provided with a cam acting on the stretching rod and which can thus determine a stretching curve as a function of time. This cam system however is very rigid and renders necessary for each new application and curve another cam with another suitable machining.
Document EP A-0,499,135 A2 discloses a machine with servo electric driven stretch rods of the type defined in the preamble of the independent claims. This machine is provided with means for determining the position of the stretch rod in their holding means channel and means for synchronizing the transmission of high pressure fluid to the preform based on the position of the stretch rod.
The aim of the present invention is to improve said known machine in particular as concerns its facilities of calibration to different moulds and preforms and to allow a high working precision and security even under changing conditions.
The machine according to the invention is characterized to this and by the fact that the control installation and its first regulating means are arranged so as to carry out a first calibration operation to determine the maximum path of the stretching rod as a function of the length of the mould cavity used, by advancing the stretching rod to the bottom of the mould cavity and by recording in the memory of the control installation the path obtained, decreased by the thickness of the bottom wall of the receptacle, and to carry out a second calibration operation to determine the path of positioning before stretching of the stretching rod as a function of the length of the preform, by advancing the stretching rod to the bottom of the preform and by recording in a memory of the control installation a value depending on the positioning path and variations in the length of the preforms.
By these characteristics, it is possible to obtain and control in a precise manner the calibration of the path of the stretching rod. These calibration operations may be made automatically and as frequently as necessary. Thus, thermal variations of the mould and all variations in the geometry of the preforms may be kept under surveillance and control. This allows to obtain a high working security and a constant high quality of the produced goods.
Favorably the first regulating means are adapted to predetermine a stretching function setting at least one parameter defining the stretching of the preform in relation to at least another parameter, advantageously the time, the second regulating means being adapted to predetermine at least one function of pressure increase and the control installation being adapted to provide a combination of said first and second regulating means.
By these characteristics, it is possible to be able to control in a precise manner the process of transformation of a preform into a receptacle or bottle. It is thus possible to pre-program and execute the stretching curves as a function of given parameters, such as time, and curves of pressure increase of the blowing air and to combine these curves so as to obtain a high quality of the products obtained. The invention permits very rapid adaptation of the machine to other uses, and other shapes and sizes of moulds and preforms. Moreover, the movements of the stretching rod are exactly controlled according to predetermined laws which can easily be modified, which permits avoiding damaging the moulding device and the preforms, obtaining a high safety during production, whilst conferring on the machine a great facility for adaptation and modification.
According to a preferred embodiment, the first regulating means of the control installation are arranged so as to pre-program the curves, functions and stretching laws having regard for the stretching path followed by the stretching rod and/or the stretching force exerted by the stretching rod as a function of time, constituting the other parameter, and then to carry out these curves, functions and stretching laws.
The stretching curves thus defined permit precise control of the formation of the receptacle.
Ideally, the control installation is arranged such that said combination of the first and second means permits programming the pre-blowing and/or blowing pressure curves as a function of the path or the position of the stretching rod and/or of the stretching force or of the resistance exerted on the stretching rod and then to carry them out.
With these characteristics, the control of stretching and the pre-blowing and blowing are carried out in such a way as to obtain receptacles having walls of high quality and precisely defined thickness.
Preferably, the first calibrating operation comprises a first step during which the stretching rod is advanced with reduced speed and stopped when it enters into contact with the bottom of the mould cavity, and a second step during which the rod is withdrawn by a first predetermined distance corresponding to the thickness of the wall of the bottom of the receptacle, the path thus obtained being recorded in the memory of the control installation.
Optimum calibration of the path of the stretching rod is thus obtained, which permits very precise production of receptacles, great safety and rapid adaptation of the machine.
Preferably, the second calibrating operation comprises a first step during which the stretching rod is advanced with reduced speed within the interior of the preform and stopped when it enters into contact with the bottom of the preform, and a second step during which the rod is withdrawn by a second predetermined distance, having regard to eventual deformation of the preform after its heating, the path thus obtained being recorded in the memory of the control installation.
These characteristics ensure reliable operation even in the case of preforms subjected to considerable deformation. Moreover, the machine automatically and rapidly adapts to all changes of dimension of the preforms.
According to a preferred embodiment, the motor is a servo-motor receiving from the control installation voltage and/or frequency and/or current signals and sending to the control installation current signals and/or signals relative to the position of the stretching rod.
Moreover, the electro-valve is preferably a servo-valve receiving from the control installation first voltage signals and sending to this installation second voltage signals.
These characteristics ensure a particularly certain and precise control of the execution of the production of the receptacles.
The invention also relates to a process for the production of receptacles of plastic material by means of a machine comprising a chassis and arranged along a transport path, at least one heating device and a device for moulding by stretching and blowing, the receptacles being formed from preforms disposed on supports, the device for moulding by stretching and blowing comprising a blowing member connected to a source of fluid under pressure by means of an electro-valve and a stretching rod adapted to be displaced by actuating means comprising a motor connected to a control installation comprising first regulating means to program and control the displacement of the stretching rod and second regulating means to program and control the pressure increase applied to the preforms, characterized by the fact that there is carried out by means of the control installation a first calibration operation to determine the maximum path of the stretching rod as a function of the length of the mould cavity by advancing the stretching rod to the bottom of the mould cavity and recording in the memory of the control installation the obtained path, decreased by the thickness of the bottom wall of the receptacle, and that there is carried out a second calibration operation to determine the path of positioning before stretching of the stretching rod as a function of the length of the preform by advancing the stretching rod to the bottom of the preform and by recording in a memory of the control installation a value depending on the positioning path and variations in the length of the preforms.
This process with a double calibration operation ensures high safety of production avoiding any damage to the moulds, the preforms, the produced receptacles, the stretching rods and other components. It permits very rapid and automated changing of moulds and preforms, without having to carry out substantial preliminary tests requiring a prolonged interruption of production.